In prior U.S. Pat. No. 4,306,966, for Heat Control Filter Element Bypass, there is disclosed a filter element for connection between a mechanism using a flow of fluid for its operation, such as an automotive automatic transmission, and a cooler for cooling the fluid after it has effected work in the mechanism and has absorbed heat. A bypass is disposed in the filter manifold or header and is normally closed by a valve operated by a solenoid controlled by a heat responsive element disposed in the flow of fluid at the inlet of the manifold or header. When the temperature of the fluid becomes excessive, the valve opens, and the fluid being returned from the cooler flows directly from the cooler to the transmission without flowing through the filtering element of the filter. As the fluid no longer flows through the filter element, which normally causes a substantial back pressure reducing the flow of fluid in the cooler return circuit, opening of the bypass greatly increases the flow of fluid through the system and consequently prevents the fluid from overheating.
Fluid filters are seldom installed in automatic transmission systems of motor vehicles such as passenger cars, trucks and buses when the vehicles are manufactured. It has been found that such filters are not required in new installations because the whole system, including the cooler, the automatic transmission casing and mechanism, and the piping between the transmission and the cooler, is carefully flushed during manufacturing and assembly such as to remove any dust, dirt, chips and the like. Most automatic transmission failures in the field are caused by overheating. Failed transmissions are either overhauled or replaced by rebuilt transmissions. When an overhauled or rebuilt transmission is reinstalled in a motor vehicle, costly filters are generally installed in the line between the cooler and the transmission, in order to trap any dirt, debris and other contaminant which may be present in the overhauled or rebuilt transmission casing and in the cores of the cooler. Although overhauled transmissions or rebuilt transmissions are often bench flushed prior to reinstalling in a motor vehicle, flushing of the cooler presents many problems and is generally neglected, and even bench flushing of the transmission itself is often subjected to shortcuts or complete neglect. Furthermore, re-assembly and re-installation of overhauled or rebuilt transmissions are evidentally not effected with the same cleanliness which normally accompanies assembly and installation of new clean parts on the assembly lines of the manufacturer's plant.
It is therefore a somewhat conventional practice to install a fluid filter in the line between the cooler and the transmission housing at the same time that an overhauled or rebuilt transmission is reinstalled in a motor vehicle. Such a filter necessarily causes a pressure drop that seriously reduces the fluid flow between the cooler and the transmission, especially as the filter element becomes partially plugged with dirt and contaminants, with the result that fluid flow to the cooler is substantially reduced and causes a tendency for the fluid to overheat. As most automatic transmission failures are due to overheating, the overhauled or rebuilt transmission may fail only after a few hours of operation. This rather common failure, due to overheating after a few hours of operation, coincides, paradoxically, with the few hours required to operate a rebuilt or overhauled transmission with a filter. This is due to the fact that any loose dirt and solid particles present in the system are trapped in the filter element during the first few hours of operation of an overhauled or rebuilt transmission.